If you can use a cap to reduce line voltage - can you use it to reduce secondary voltage too?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,490
I've seen places where caps have been used to reduce AC voltages from 277 down to 120 volts. I'm wondering if the same principal can be applied to transformers - particularly, their secondary output sides.

Suppose - and this is all this question is addressing, a supposition - suppose you have a transformer with a secondary output of oh, lets say 48VAC. Can you use a cap to further reduce that voltage to something like, let's just pick a number - 10VAC? If the principal can be applied to line voltage then shouldn't it be applicable to the secondary voltage?

I'm just asking out of curiosity. Have no plans for any such project at present.
 

jpanhalt

Joined Jan 18, 2008
10,913
If these are the sort of transformerless supplies to which you refer (just examples):

1607550593927.png1607550705438.png

Yes, they will work on the secondary as well. Why do it?

Edit: Grammar and spelling.
 
Last edited:

Ian0

Joined Aug 7, 2020
1,096
Absolutely, it just acts as a series impedance - the same as putting a resistor in series. Because the frequency is constant, the impedance is constant at 1/(2*pi*f*C).
It is most commonly seen as a way to make a low-current DC supply referenced to mains, without the heat a resistor would generate as @jpanhalt just posted whilst I was writing it. (Thank you for the diagram, it saved me the bother of drawing it)
It becomes the same circuit as a Cockcroft-Walton multiplier, but the series capacitor is effectively too small.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,490
Why do it?
I don't know why. Just a thought. What if I had a transformer in my stock and it was too high a voltage on the secondary. Just wondered if throwing a cap either in series or in parallel, however it's done, as a quick and dirty way of limiting the AC voltage.

Like I said, I have no projects in mind for this.
 

Audioguru again

Joined Oct 21, 2019
2,353
The capacitor does not reduce the voltage, instead it reduces the current and the voltage divider of the capacitance reactance feeding the 5V zener diode with reduced current reduces the voltage.
 

jpanhalt

Joined Jan 18, 2008
10,913
I don't know why. Just a thought. What if I had a transformer in my stock and it was too high a voltage on the secondary. Just wondered if throwing a cap either in series or in parallel, however it's done, as a quick and dirty way of limiting the AC voltage.

Like I said, I have no projects in mind for this.
It depends on how much current you need, what inefficiency you can tolerate and how stable a voltage. Some other options: linear voltage regulator, buck regulator, voltage divider, autotransformer (Variac).
 

Ian0

Joined Aug 7, 2020
1,096
It's a handy way of making "extended rails" for audio amplifiers, i.e. if the main supply is +/-55V then this circuit can make it into +/- 65V at low current by adding 10V to either rail.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,490
Like I've already said, this isn't a project. No "Need", just curiosity.

I'm messing with a low voltage transformer with a single diode rectified into an LM317 and using that to run a small motor. The setup already is working, 6.3VAC, filtered to 8.9VDC. IF - and this is hypothetical, I want a slightly lower voltage, say I only want between 6 and 7 volts, low current (milli-amp range - again, nothing specific) wondering if a cap of some value would produce the speculative result.
 

crutschow

Joined Mar 14, 2008
25,980
If you have an LM 317 why not just make that adjustable?
He's referring to the LM317 input voltage, not output voltage.
wondering if a cap of some value would produce the speculative result.
Not for that circuit.

A capacitor reduces the AC voltage similar to a resistor in series, except it doesn't dissipate power.
So a series capacitor at the transformer output would reduce the AC voltage to a bridge rectifier as determined by the current being drawn and the capacitance.
This could help reduce the dissipation in a series regulator, such as the LM317, at higher currents.

But note that the capacitor must be in series with the normal AC bipolar signal to work (equal plus and minus current).
Thus it can't be in series with the transformer winding if it's going to a half-wave rectifier, as you have.
The capacitor will just charge to the peak AC voltage and then stop conducting.
It will work okay with a bridge rectifier, since in that case, the AC conducts in both directions.
 

Wolframore

Joined Jan 21, 2019
2,176
lm317 can take has max input voltage of 40V, why do we need to reduce input voltage?

I’ve used it with a bridge rectifier when the expected current draw is steady, when you have fluctuating current it doesn’t work well
 

Ian0

Joined Aug 7, 2020
1,096
Also worth bearing in mind is that it forms a high-pass filter, so any high-frequency interference on the mains comes through loud and clear!
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,490
He's referring to the LM317 input voltage, not output voltage.
Not for that circuit.

A capacitor reduces the AC voltage similar to a resistor in series, except it doesn't dissipate power.
So a series capacitor at the transformer output would reduce the AC voltage to a bridge rectifier as determined by the current being drawn and the capacitance.
This could help reduce the dissipation in a series regulator, such as the LM317, at higher currents.

But note that the capacitor must be in series with the normal AC bipolar signal to work (equal plus and minus current).
Thus it can't be in series with the transformer winding if it's going to a half-wave rectifier, as you have.
The capacitor will just charge to the peak AC voltage and then stop conducting.
It will work okay with a bridge rectifier, since in that case, the AC conducts in both directions.
This is all interesting information. And yes, the notion behind doing so is to reduce the amount of voltage the 317 has to dissipate as heat. It's just a purely speculative question, but yes, the BR makes sense too. Thank you.
lm317 can take has max input voltage of 40V, why do we need to reduce input voltage?

I’ve used it with a bridge rectifier when the expected current draw is steady, when you have fluctuating current it doesn’t work well
Again, this is just an exercise in theory and thought.

Back in the early 80's I serviced emergency lighting equipment. I came across an exit sign that had power requirements of 120VAC but was wired into the store's 277VAC. They used a capacitor to drop the voltage. And while memory has faded, I THINK the cap had three wires. It was an entirely new concept for me, one I've never understood or even taken the time to try to. Now I'm building this project and got to wondering if adding a cap to the circuit I could limit the amount of voltage the 317 would have to dissipate. As it is there's not significant heat being dissipated now. The motor is rated for 5.9 volts I THINK. I've never seen markings on a motor like this.

It says:
D/V 5.9

Nevertheless, using the 317 I'm able to moderate the RPM of the motor. The ONLY work it's doing is rotating a mirror that is mounted on the end of the shaft. The mirror is slightly tilted off of perpendicular to the shaft. It's meant to reflect a beam in a circular pattern and nothing else. So the load is light. I haven't been able to get any more information on the motor other than this.

At least I'm learning a few things about capacitance and dv/dt. Some of the math is mud to me, so - - - . But thanks for all the informative responses. Perhaps sometime soon I'll throw up a rig and take some measurements and discover what happens. And it will be done with only low voltages.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
5,490
The motor has no load on it other than just spinning a mirror. When power is on the motor spins. When power is lost the motor coasts to a stop. There are no driven gears or anything else that can hang the motor up. For clarity:
1607614786358.pngRF-310T-11400
 
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